Abstract Background Infantile oxalosis, the most devastating form of primary hyperoxaluria type 1 (PH1), often leads to end-stage renal disease (ESRD) during the first weeks to months of life. Case-diagnosis Here, we report the outcome of the therapeutic use of Oxalobacter formigenes (Oxabact OC5; OxThera AB, Stockholm, Sweden) in a female infant with PH1 who exhibited severely elevated plasma oxalate (Pox) levels, pronounced nephrocalcinosis, anuretic end-stage renal disease, and retinal oxalate deposits. Following the diagnosis of PH1 at an age of 8 weeks, a combined regimen of daily peritoneal dialysis, daily pyridoxine treatment and hemodialysis (3 times a week) was unable to reduce the pronounced hyperoxalemia. After the addition of Oxalobacter formigenes therapy to the otherwise unchanged treatment regimen, Pox levels first stabilized and subsequently declined from 130 μmol/L to around 80 μmol/L. Nephrocalcinosis and retinal deposits stabilized. Oxalobacter formigenes treatment was well-tolerated and no related adverse events were observed. The patient showed nearly age-appropriate growth and development and received successful combined liver-kidney transplantation at the age of two years. Conclusions Treatment with O. formigenes combined with intensive dialysis led to reduction of Pox, stabilization of systemic oxalosis, and improvement in the clinical disease course. O. formigenes treatment may be an option for reduction of oxalosis in infantile patients with insufficient response to conservative treatments until combined liver-kidney transplantation can be performed.